High Early Discharge & Orifice Design in OSD Systems

The smallest part of the system does all the work

An on-site detention system can involve tens of cubic metres of concrete, but the component that actually does the flood mitigation is a steel plate with a hole in it. The orifice plate is what limits your site’s outflow to the permissible site discharge (PSD); the tank is just the buffer that holds water while the orifice meters it out. Understanding how the orifice works — and where the high early discharge (HED) arrangement fits in — explains most of what a council assessor is checking on your OSD drawings.

How an orifice controls flow

An orifice is deliberately simple: a precisely sized circular opening, usually cut in a plate fixed inside the discharge control pit at the tank outlet. The physics is the useful part for builders to grasp:

• Flow through the orifice depends on the depth of water above it. The deeper the water in the tank, the higher the pressure head, and the faster water pushes through the hole.
• The orifice is sized so that at maximum head — tank full to the design level — the outflow just reaches the PSD. At lower water levels, outflow is less than the PSD.
• The relationship is governed by the standard orifice equation: discharge rises with the square root of the head. Double the depth doesn’t double the flow.

The diameter is calculated, not chosen — it falls out of the PSD and the geometry of the tank, following the council’s adopted handbook method and design rainfall consistent with Australian Rainfall & Runoff. This is also why field changes are so dangerous: enlarge the hole and you exceed your PSD; reduce it or mount the plate at the wrong level and the tank may not drain as designed.

The problem orifices create — and why HED exists

A plain orifice has a quirk: it’s least effective when the tank is nearly empty. Early in a storm, with little depth over the orifice, outflow is well below the PSD — the system is “wasting” discharge capacity it’s legally allowed to use. That means the tank fills earlier and you need more storage volume to survive the design storm.

The high early discharge (HED) arrangement fixes this. In a typical HED setup, a riser or chamber arrangement in the discharge control pit allows the system to discharge at close to the full permissible rate from early in the storm, rather than ramping up slowly with depth. The classic configuration uses a small HED chamber that fills quickly and drives head over the orifice almost immediately, with the main storage coming online as the event continues.

The payoff is direct: because the outlet works at near-PSD for more of the storm, the storage volume needed is smaller. Council handbooks that adopt HED arrangements — including the methodology descended from the UPRCT On-Site Stormwater Detention Handbook used across western Sydney — generally reflect this in their site storage requirement (SSR) calculations. On a tight lot, the difference in tank size can be the difference between a workable design and one that doesn’t fit.

The supporting cast in the discharge control pit

The orifice never works alone. A compliant discharge control pit typically includes:

• Trash screen. A removable screen protecting the orifice from leaves, litter and debris. A blocked orifice turns your detention tank into a pond, so councils are particular about screen area, clearances and removability for cleaning.
• Overflow / surcharge path. Storms bigger than the design event must escape safely — over a weir, through a surcharge grate or along a designed overland flow path — without flooding the building or the neighbour.
• Access. Lids and step irons (or equivalent) so the pit and screen can actually be inspected and cleaned after handover.
• The plate itself. Durable material (commonly stainless or galvanised steel), fixed — not glued or improvised — with the orifice diameter and invert level matching the approved drawings exactly.

What goes wrong on site

In our certification work, the same orifice problems come up repeatedly:

• Orifice plate missing entirely — the pit was built, the plate never installed.
• Wrong diameter, or a roughly cut hole that doesn’t match the calculated size.
• Plate set at the wrong level, changing the head relationship and the storage behaviour.
• No trash screen, or a screen that can’t be removed for cleaning.
• HED arrangement simplified out during construction “because it looked complicated”.

Every one of these shows up at the works-as-executed survey and certification stage — and every one is cheaper to fix on the drawings than in built concrete. Build the discharge control pit exactly as drawn, and ask the designer before substituting anything.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW — orifice sizing, HED arrangements and certification included. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us today.